Protective device



Oct l0, 1933- E. R. TAYLOR l PROTECTIVE DEVICE Filed Nov. 24,'1931INVENTOR 12112259@ BY QL ATTORNEY Patented oct. 1o, 1933 lPATENT:OFFICE]i This invention relates to telephone systems and moreparticularly tothe vassociation of arrangements with such systems-*forpreventing acoustic shocks to operators and subscribers connected tosuch systems.

Certain types of telephone lines are exposed to lexcessive voltages fromexternal sources such as adjacent power lines; This results not'only indamage to equipment-but also in serious acoustic ,w

shocks to telephone operators-andlsubscribers Heretofore, arrangementshave beenr provided izo-*protectthe operators by placing a two-elemeritcold vcathode gas-filled tube in shunt with the operators receiver. -Theconstants of atube i5- ofv thistype are such thatithe gas ionizes whenthe voltage across the receiver rises to harmful values.- Thisionization of the tube causes the i impedance of theV tube to begreatlyreduced and devices couldnot be used on lines uponf whichv 'henceeffectivelyl places a short circuit-across the *2o receiverithroughoutthe duration of the'excessive voltage, eiectively'pre'venting acousticshockto the operator; Such Ian arrangement has `the objectionthat itdoes not protectlthe subscriber and'requiresV protectiveI` apparatus foreachop- .2.5,

eratorwho has access to theeXposed line. This mightresult in`theprovision of protective approposedwhereby the protective apparatuswould be"" placed on a telephone line'in such a manner that both theoperator and 'subscriber would b e protected and so that the'number yofprotective devices would vbe in direct proportion to the L35;

number of exposed lines. vSuch devices mightrbe shunted' across ythetelephone `line near the' 'pointfwhere rit/:enters the office.found'that such devicesfmust operate on Voltagesga's low as 10 or115volts. nThesedevices '40' would vhave `thedisadvantage that theyl wouldnotipermit ZO-cycle or 135-cycle ringing ordirect current telegraph,normally operating` at voltages inexcess` of 1 5 volts, soit isobviousfthat such such' signaling is superimposed. i

The arrangements of the invention are particularly designed to overcomethe above mentioned diiculties and, when associated with a telephoneline, will not only prevent acoustic shocks but will allow the use ofsignaling currents such as 20 or 135-cycle signaling currents,

which have operating voltages in excess of vl5 volts. The operation ofthe arrangements of the invention isv based on the fact that thesignaling currents, which should not be interfered'V .It-has been`r fwith but'which may have potentials greater than 110 `vo1ts,-are all atcomparatively low frequencies,

for example, below 150 cycles, whereas the eX- `cessive voltages whichare to be guarded against normally ,contain many components havinghigher 'frequencies as well as low frequencies. Other objectsl andfeatures ofthe invention will `appear more fully'from vthe detaileddescription thereof hereinafter given. `In the drawing is `shown acircuit diagram embodying apreferred vform of the invention.

Inthe drawing is shown a telephone line` L which may be exposed toexcessive voltages'from external sources such'as adjacent power lines.This line might connect the two telephone sets l andV 3. Associated witheach telephone set lis the apparatus such as 2 and v4,ffor Asending andreceiving signaling `frequencies,below' 150 cycles.Vv Bridged across`the telephone line by the circuit 5 would be a high pass filter 6,pass- 75 ing only frequencies above cyclesl Associated'with the filter 6by means of ther transformer? would be the input circuit of agasfilledm'tube 8. It is a well known property of4 tubesof this typethat 'as'long Vas the grid is 80' sufliciently negative with respect tothe cathode', ionizationdoes not take place and the plate-tocathodeimpedance is veryv high. However, when this negative potential isreduced to a value even slightly below ther critical value,ionizationtakes g5 place, permitting'large currents to flowv between thecathode gand plate A,and reducing the jplateto-cathode impedance tofverysmall values. It

is also well kno'wn`that"this` ionization will per-V sist without regardto subsequent grid potentials 90,

gu'ntil the plate Ypotentiall is `very materially reducedforinterrupted. The plate circuit of this tube'includes the primary windingof a .transl former 9, the secondary of which is shunted across thetelephone line in the manner shown. The Vconstants of transformer v9 aresuchI that .when the impedance across the primary .Winde ing is verylarge theshunting eiect of the sec# londary Winding on 'the telephoneline willibe very small, but when the impedance .across the primary issmall the shunting effect ondarywill be large. The plate circuit of vthetubealso includes a resistance 11, the winding of a-relay 10, and anormally made contact of such relay, all in series." The purpose of theresistance is to limit the current Iiowduring ionization in order tosafeguard theV tube. The relay 10 also has a set of normally opencontactsand the armaof [the sec;

ture and contacts are so arranged that the nor mally open contacts arealways closed when the normally closed contacts are open. Thealternating current impedance of the resistance and relay Wniding isshunted by a condenser 12. The normally open contacts are hunted acrossthe telephone line.

When an excessive voltage is applied to the telephone line L, thehigh-frequency components thereof pass through the filter and reduce thenegative potential of the grid, causing the gas in the tube to beionized and hence changing the impedance across the primary of theoutput transformer 9 from a very high toa very low value. Thiseffectively places an electromagnetic short circuit on the telephoneline, preventing the production of acoustic shock and restoring the gridpotential to normal. Ionization continues in the tube and the resultantflow of plate current causes the operation of relay 10, which places amechanical short circuit across the telephone line and then opens theplate circuit of the tube, causing de-ionization to take place and thetube to return to normal. The relay 10 then releases, first closing theplate circuit and then removing the mechanical short circuit from thetelephone line.

If the excessive voltage has not been dissipated in the meantime, theabove cycle of operation will be repeated until such dissipation hasbeen completed. `The period of the cycle may be changed by changing theoperate and release times of the relay, since the operate time of theelectromagnetic short circuit is independent of relay times. It ispointed out that the t gas-lledvtube 8 will operate only on one-halfprising a circuit bridged across said line, a filter in said circuitdesigned to pass currents of frequencies above those ordinarily used forsignaling overl said line and to attenuate and practically extinguishcurrents of such signaling frequencies, a gas filled thermionic tubehaving its input connected to said circuit, and a transformer having itsprimary winding in the plate circuit of said tube and its secondarywinding in shunt across said telephone line.

' 2. A transmission line and a protective device associated therewith,said protective device comprising a circuit bridged across said line, afilter in said circuit designed to pass currents of frequencies abovethose ordinarily used for sig--A naling over said line and to attenuateand practically extinguish currents of such signaling frequencies, a gaslled thermionic tube having its input connected to said circuit, andmeans controlled by said tube when operated for placing a magnetic shortcircuit across said telephone line.

3. A transmission line and a protective device associated therewith,said protective device comprising a circuit bridged across said line, afilter in saidcircuit designed to pass currents of frequencies abovethose ordinarily used for signaling over said line and to attenuate andpractically extinguish currents of such signaling frequencies, a gasfilled thermionic tube having its input connected to said circuit, and atransformer having its primary winding in the plate circuit-of said tubeand its secondary winding in rshunt across said telephone line, theconstants of said transformer being such that when the Aimpedance acrossthe primary is large the shunting effect of the secondary on thetelephone line will be small and when the impedance across the primaryis small the shunting effect of the secondary will be large.

4. A transmission line and a protective device associated therewith,said protective device comprising a circuit bridged across said line, afilter in said circuit designed to pass currents of frequencies abovethose ordinarily used for signaling over said line and to attenuate andpractically extinguish .currents of such signalingfrequencies, a gasfilled thermionic tube having-its input connected to said circuit, atransformer having its primary winding in series in the plate circuit ofsaid tube and its secondary winding connected in shunt across saidtelephone line, whereby when said tube operates a magnetic short circuitwill exist across said line, a relay in series in the plate circuit ofsaid tube, and means controlled by said relay for closing a mechanical`short circuit across said telephone line and for opening said platecircuit.

5. A transmission line and a protective de vice associated therewith,said protective device comprising a circuit bridged across said line, afilter in said circuit designed to pass currents of frequencies abovethose ordinarily used for signaling over said line and to attenuate andpractically extinguish currents of such-signaling frequencies, a gasfilled thermionic tube having its input connected to said circuit, atransformer having its primary winding in series in the plate circuit ofsaid tube and its secondary winding connected in shunt across saidtelephone line, whereby when said tube operates a magnetic short circuitwill exist across said line, a` relay having its Winding connected insaid plate circuit in series with a normally closed contact of saidrelay, anda set of normally open contacts for said relay for controllinga mechanical short circuit across said telephone line, the arma,- tureand contacts of said relay being so, arranged that the normally opencontacts` are always closed when the normally closed contacts are open.

EDMUND R. TAYLOR.

